A greater magnitude of gut microbial product translocation (MT) and gut permeability in women may have important clinical implications for risk of developing autoimmune diseases as well as resistance to infection and response to vaccines. Increases in the inflammatory CD14+CD16++ monocyte subset and enhanced monocyte activation in women may alter effector responses of these cells to bacterial and viral infection and to TLR agonists, resulting in an increased susceptibility to autoimmune disease in women compared to men. TLRs and TLR-signaling downstream cytokines (e.g., IFN-?IL-6, IL-1?TNF-?play a key role in the pathogenesis of autoimmune diseases, and treatments against specific cytokines or TLR inhibitors are effective in SLE patients and murine models. We hypothesize that sex hormone mediated permeable gut and resultant increased magnitude of MT are fundamental drivers for pro-inflammatory cytokine production monocyte activation and sex bias in autoimmune disease. Understanding these mechanisms is likely to provide insights crucial for the development of sex-specific interventions for autoimmune disorders and other diseases sharing sex differences such as Hepatitis C and HIV. Sex-based differences in MT provide a model for analysis of the effects of MT and altered innate immune responses on adaptive immunity and disease pathogenesis in vivo. If our hypothesis is correct-that heightened MT or altered TLR responsiveness in healthy women results in heightened persistent immune activation and pro-inflammatory cytokines, and that heightened inflammation results in enhanced susceptibility and heightened disease activity in SLE-, a therapeutic strategy (e.g., mucosal protector/microbes) may help reverse high levels of persistent inflammation, thereby helping to reestablish normal immunity/tolerance in autoimmune diseases. SPECIFIC AIM 1: Determine the role of sex hormones in gut permeability and the magnitude of microbial product translocation in healthy controls and patients with SLE in vivo and in vitro. Our preliminary data show that premenopausal women have higher plasma levels of total bacterial rDNA, a marker of MT in vivo, compared to men and postmenopausal women. To understand the impact of microbial products from plasma and the gut on systemic immunity and SLE, we will analyze microbial genetic potential by metagenomic sequencing in plasma. SPECIFIC AIM 2: Determine the role of sex hormones in TLR4 responsiveness and monocyte activation. We hypothesize that gut permeability and heightened MT result in monocyte activation in women. We will assess monocyte subset differentiation and potential drivers on monocytes. We will analyze the relatedness/correlation between MT, TLR4 responsiveness, monocyte activation, levels of sex hormones, and SLE disease activity. SPECIFIC AIM 3: Determine the impact of estrogen on gut permeability and monocyte activation in a cohort of postmenopausal women. In this aim, we will analyze the magnitude of MT and monocyte activation in 40 postmenopausal women before and after receiving hormone replacement therapy. By defining mechanisms of sex differences in gut mucosal integrity, MT and systemic immunity and by demonstrating a relationship among gut permeability, MT, TLR4 responsiveness and monocyte activation in vivo, we will establish the plausibility of this model in the pathogenesis of SLE. These findings have broader implications including vaccine responses, infectious disease risk, and risk of other autoimmune diseases.